1. Field of the Invention
The present invention relates to a manufacturing method of semiconductor device having thin film transistor, particularly to a technique for forming crystalline semiconductor film, which comprises active layer of thin film transistor.
2. Related Art
As a forming method of active layer on thin film transistor (Thin Film Transistor: hereinafter, referred to as TFT), a technique, in which an amorphous semiconductor film is formed on a substrate having an insulation surface, and then, crystallize the same in a manner of laser annealing or heat annealing, has been developed.
The laser annealing is known as a crystallization technique, in which a high energy is given to an amorphous semiconductor film only without allowing the temperature of a glass substrate to rise too high, and thereby, the amorphous semiconductor film is crystallized. Particularly, excimer laser is a typical laser, which oscillates short-wave length light of 400 nm or less in wavelength, has been used since the laser annealing was developed. The laser annealing is carried out in such a manner that a laser beam is processed by means of an optical system so as to be shaped into a spot-like form or a linear form at a surface to be irradiated, and the surface to be irradiated on the substrate is scanned by the processed laser beam; i.e., irradiation position of the laser beam is shifted with respect to the surface to be irradiated.
However, the crystalline semiconductor film, which is prepared by means of the laser annealing, comprises an aggregate of a plurality of crystal grain (ordinary crystal grain size, which is prepared by means of a conventional excimer laser crystallization, is approximately 0.1-0.5 μm), and, the position and the size of the crystal grain are not even.
As for the TFT, which is prepared on a glass substrate, in order to isolate elements, since the crystalline semiconductor film is formed being separated into island-like patterns, it was impossible to form crystal grains at specified positions and sizes. Accordingly, it was almost impossible to form channel-forming areas with a monocrystal semiconductor while eliminating the influences of the crystal grain boundary.
The interface (crystal grain boundary) of the crystal grain is an area where the translational symmetry of the crystal is decayed. It is known that, due to the influence of the recombination center or trapping center of the carrier, or the influence of the potential barrier in the crystal grain boundary caused from the crystal defect or the like, the current transport characteristics of the carrier is decreased, and as a result, the OFF-current increases in the TFT.
A technique called as super lateral growth, by which, compared to the crystal grain size via conventional excimer laser crystallization, a larger grain size can be formed, is known. A detailed description of the technique is disclosed in “On the super lateral growth phenomenon observed in excimer laser-induced crystallization of thin Si films, James S. Im. and H. J. Kim, Appl. Phys. Lett. 64(17), 25 Apr. 1996, pp2303-2305”.
In the super lateral growth, a portion, where the semiconductor is melted completely due to the irradiation of a laser beam, and a portion where the solid-phase semiconductor area remains, are formed, and then, the crystal growth begins around the solid-phase semiconductor area as the crystal nucleus. Since it takes a certain period of time for nucleation to take place in the completely melted area, during the period of time until the nucleation takes place in the completely melted area, the crystal grows around the above-described solid-phase semiconductor area as the crystal nucleus in the horizontal direction (hereinafter, referred to as lateral direction) with respect to the film surface of the above-described semiconductor. Therefore, the crystal grain grows up to a length as long as several tens of times of the film thickness. For example, with respect to the silicon film thickness of 60 nm, a lateral crystal growth of 1 μm-2 μm in length takes place. Hereinafter, the phenomenon will be referred to as super lateral growth.
In the case of the above-described super lateral growth, although a relatively large crystal grain can be obtained, the energy intensity area of the laser beam, where the super lateral growth is obtained, is much stronger than the intensity that is used in ordinary excimer laser crystallization. Also, the range of the energy intensity area is extremely narrow. From the viewpoint of the position control of the crystal grain, it is impossible to control the position where a large crystal grain is obtained. Further, the area other than that of the large crystal grain is the microcrystal area where countless nucleation has taken place, or the amorphous area; the size of the crystals is not even and the roughness of the crystal surface is extremely large.
Accordingly, the irradiation condition, which is generally used in manufacturing of semiconductor devices, is the condition where even crystal grain size of approximately 0.1 μm-0.5 μm can be obtained.
Further, according to “Sequential lateral solidification of thin silicon films on SiO2, Robert S. Sposili and James S. Im, Appl. Phys. Lett. 69(19), 4 Nov. 1996, pp2864-2866”, James S. Im et al. disclosed a Sequential Lateral Solidification method (hereinafter, referred to as SLS method), in which, by controlling artificially, the super lateral growth can be obtained at a desired position. According to the SLS method, an excimer laser beam of pulse oscillation is irradiated to a material via a slit-like mask. According to the SLS method, the crystallization is carried out while the relative position between the material and the laser beam is displaced, at every shot, by a distance (approximately 0.75 μm), which is roughly equivalent to the length of the crystal formed via the super lateral growth; thereby, the crystal is allowed to grow by means of artificially controlled super lateral growth.
As described above, by using the SLS method it is possible to prepare crystal grains at desired positions under artificially controlled conditions, in a manner of the super lateral growth. However, the SLS method has the following problems as described below.
First of all, the problem is that the substrate processing efficiency (throughput) is insufficient. As described previously, in the SLS method, the crystallization distance per shot of laser beam is approximately 1 μm. Accordingly, it is necessary that the relative shift distance (feed pitch) of the laser beam between the beam spot on the material surface and the material substrate is 1 μm or less. In the conditions adopted in the ordinary laser crystallization using a pulse oscillation excimer laser, feed pitch per shot of laser beam is several 10 μm or more. Needless to say, under such conditions, the crystal peculiar to the SLS method can not be prepared. In the SLS method, although a pulse oscillation XeCl excimer laser is used, the maximum oscillation frequency of the pulse oscillation XeCl excimer laser is 300 Hz. Under such conditions, only the crystallization area of maximum 300 μm or so is processed in distance in the scan direction of the laser beam. At the processing speed as described above, in the case of a large size substrate such as, for example, 600 mm×720 mm in dimension, with the SLS method, it takes an extremely long period of time to process one sheet of substrate.
The fact that it takes a long processing time per sheet of substrate is not only the problem of time and cost. That is to say, practically, in the case of crystallization of an amorphous semiconductor film, the surface processing thereof is critical. For example, in the case that laser irradiation is carried out after removing natural oxide film with dilute hydrofluoric acid or the like as a pre-processing, in the surface of the substrate, compared to the area where is subjected to the laser irradiation at the first, there is a possibility that natural oxide film grows again in the area where is subjected to the laser irradiation at the last. In this case, amount of Carbon, Oxygen, Nitrogen, or amount of contamination impurities such as Boron or the like, which is taken in the finished crystal, may vary within the surface of the substrate resulting in an unevenness of the transistor characteristics within the surface of the substrate.
Secondly, such a problem that the optical system tends to be complicated remains in the conventional SLS method. It is necessary to incorporate a mask, which processes slit-like the configuration of the laser beam power at the substrate surface, into the optical system. Ordinarily, film thickness of an active layer silicon, which is used for polycrystalline silicon thin film transistor, is several tens nm or more. When a pulse oscillation excimer laser is used, the laser energy density necessary for the laser crystallization is at least 200 mJ/cm2 (as a typical example, for an amorphous silicon film of 50 nm, approximately 400 mJ/cm2 with an XeCl excimer laser of 30 nsec pulse width). In the SLS method,there is a super lateral growth condition, which is the optimum for a further slightly stronger energy density area. It is difficult to prepare a slit-like form mask, which is capable of enduring such strong laser energy density. In the case of the mask of metal material, being subjected to the pulse laser beam irradiation of a strong energy density, the temperature of local film is raised and cooled down rapidly. As a result, it may cause a peeling or decay of configuration of a minute pattern due to a long period of use (as for photo lithography for resist exposure, although a hard mask material such as chromium or the like is used, since incomparably weaker energy density than the laser energy density necessary for silicon crystallization is used, there is no problem such as peeling or a decay of configuration of the minute pattern). As described above, there is such a factor that optical system becomes complicated resulting in a difficulty of maintenance of an apparatus in the conventional SLS method. Further, in order to carry out the super lateral growth, it is necessary to make the spatial beam power profile of the laser beam sharp (to eliminate attenuation area of the optical power between the irradiation area and the non-irradiation area of the laser beam). In the conventional SLS method, since the beam necessary for the super lateral growth cannot be condensed using the ordinarily optical system only, the excimer laser is used. Accordingly, it is understood that a slit-like mask is required to shield the laser beam partially.
The object of the invention is to solve the above-described problem, and further, to increase the positional control of crystal grains in accordance with the layout of the TFT, and simultaneously, to increase the processing speed of the crystallization process. More particularly, it is an object of the invention to provide a manufacturing method of semiconductor devices, which is capable of forming large size crystal grains successively in a manner of super lateral growth under an artificial control, and capable of increasing the substrate processing efficiency in the laser crystallization process.
Further, the invention provides a manufacturing method of semiconductor devices which is capable of forming large size crystal grains successively in a manner of super lateral growth under an artificial control, and capable of increasing the substrate processing efficiency in the laser crystallization process, as well as to provide a manufacturing method using a convenient laser irradiation method which does not need to incorporate a mask, which processes the configuration of the laser beam power into a slit-like shape on the substrate surface, into an optical system unlike the conventional SLS method.